The present invention relates to semiconductor fabrication. More particularly, the present invention relates to photomasks utilized in semiconductor fabrication lithography.
Lithography is conventionally used in the fabrication of semiconductor devices. In optical lithography, a photosensitive film, a photoresist, is patterned by a photomask. Photoresist is exposed to light from a light source through the photomask. An etch or implantation of the wafer may be performed based upon the photoresist pattern.
The photomask has areas composed of an absorbing layer in a pattern which corresponds to the desired circuitry for the device. The absorbing layer can be chromium, or some other opaque or partially transmitting material. Alternative materials to chromium, such as, molybdenum disilicide (MoSi2) have also been pursued in certain photolithographic applications. These alternative materials have been pursued primarily for their better processing capability and for use as attenuated phase-shifting masks, rather than for use as opaque materials.
FIG. 1 illustrates a conventional photomask 110 and photoresist 130 as used in lithography. The photomask 110 may comprise fused silica 112. On the bottom side of the photomask 110 are areas 120. Areas 120 are generally an opaque material such as chromium. The photomask 110 is treated with light (e.g., ultraviolet light) from a light source (not shown). The light shines through the photomask 110 where there are no areas 120. The light is reflected or absorbed where there are areas 120. Some of the light that passes through the photomask 110 continues through a lens 140, which projects an image of the mask pattern onto the photoresist 130, which undergoes a chemical reaction when exposed to light. Portions 150 of the photoresist 130 are exposed to the light while portions 160 of the photoresist 130 are ideally not exposed to the light.
However, approximately 4-5% of the light is lost through reflections off each of the two surfaces of the photomask, as illustrated by arrows 170 in FIG. 1. Light reflected from the substrate or wafer can be transmitted back through the lens and back to the photomask 110. A portion of this light is then reflected by the photomask 110 back to the photoresist 160 on the wafer, as shown by arrows 180. Such reflected light is not part of the integrated image of the photomask 110, and it can degrade the quality of the light pattern in the photoresist 160.
In addition, although conventional absorbing materials, such as, chromium materials, partially reflect ultra-violet (UV) light and partially absorb UV, light they are primarily absorbers. For example, for chromium, approximately 30% of light at the interface between area 120 and fused silica 112 is reflected. The remaining 70% of the light is absorbed by area 120.
Absorption of a significant amount of light by area 120 can heat photomask 110, thereby resulting in substantial registration errors. Calculations performed by the University of Wisconsin predict a nearly eighty nanometer (nm) registration error due to light absorption by the absorbing layer (area 120). Approximately fifty percent of this error cannot be corrected using conventional means while the remaining fifty percent of this error can only be corrected by reducing exposure tool productivity. Anti-reflective coatings utilized on absorbing layers in conventional semiconductor fabrication processes can exacerbate the heating problem because the conventional anti-reflective coatings reduce reflection through absorption.
Accordingly, what is needed is a system for and method of improving the transmission of light through photomasks. The method and system should decrease the loss of light due to reflections and decrease undesired exposure of portions of the photoresist. Further, there is a need for a method of and a system for reducing heat absorbed by the photomask. Even further, there is a need for a method of and a system for decreasing registration errors associated with the photomask.
One exemplary embodiment relates to a method of forming a photomask. The photomask is utilized in an integrated circuit fabrication process. Light is transmitted through the photomask. The method includes providing a photomask substrate and applying an opaque material to at one side of the photomask substrate. A reflection of the light at an interface between the substrate and the opaque material is at least 45% and absorption by the opaque material is reduced with respect to conventional photomasks.
Another exemplary embodiment relates to a system. The system includes a photomask substrate, an opaque material, and at least one anti-reflective coating. The opaque material is on one side of the photomask substrate. At least one anti-reflective coating is on an other side of the photomask substrate. A reflection at an interface between the opaque material and the substrate is 45% or greater.
Yet another exemplary embodiment relates to a method of transmission of light through a photomask. The method includes steps of providing a photomask substrate, applying an anti-reflective coating to a first side of the photomask substrate, and applying an opaque coating to a second side of the photomask substrate. The opaque coating includes molybdenum.
Still another exemplary embodiment relates to a system for use with light in a photographic process. A system includes a photomask substrate, and an opaque material on one side of the photomask substrate. The light has a reflectance of 45% or greater at an interface between the photomask substrate and the opaque material.
Yet another exemplary embodiment relates to a system including a photomask substrate and a opaque means for defining a pattern on the substrate, reducing light absorption.